The invention relates to a rotational speed superposition device as well as to a method for operating a rotational speed superposition device.
Such motor vehicle steering mechanisms with superposition gearing are employed in modern steering systems for free-riding or non-trackbound motor vehicles. The electromagnetic auxiliary drive provided in such superposition gearings serves for superimposing rotations of the superposition gearing due to the auxiliary drive onto the steering interventions carried out through the operating person on the control wheel (steering wheel). In this manner the relation of the turning speed of the motor vehicle wheels in comparison to the turning speed of the steering wheel becomes adjustable and can be adapted to the particular driving situation. Thus, for example when parking-in, small angular changes on the steering wheel are intended to lead to large angular turnings at the wheels, while during fast freeway travel even large angular changes on the steering wheel are intended to lead only to small angular turnings at the wheels. Possibly even the automated parking-in should be made possible without turning of the steering wheel. In this manner, via the auxiliary drive, for example, corrective action can simultaneously be exerted onto steering movements, which are erroneous or too vigorous, by the motor vehicle driver. Such steering movements requiring correction can occur, for example, in overreactions of the driver in hazardous situations.
The function of a rotation speed superposition, which can also be referred to as rotational angle superposition, during the wheel-steering process is consequently decoupled compared to a force coupling or torque coupling such as serves in a power steering to facilitate the control process.
Various such devices are already known within prior art. Apart from the application of planetary gearings for the superposition of the rotational angle or rotational speed, strain wave gearings, also known by the term harmonic drive gearing or pulsator gearing, are also utilized.
In such gearings for the superposition of the rotational angle or rotational speed the input drive shaft and the output drive shaft as well as an output shaft of an auxiliary drive are each connected with a gearing member of the superposition gearing. Such gearing members are for example a toothed wheel or a planetary carrier. The superposition takes place through specific gearing structures, to some extent comprised of further gearing elements, thereby that the rotational speed of the output shaft of the auxiliary drive and the input shaft are added to appropriately predetermined rotational speed transmissions and are transferred onto the output drive shaft of the device. In addition to the rotational speeds, torques are correspondingly also transferred. A corresponding reinforcing of the torques on the auxiliary drive, and on the input and output shaft, is therefore required since otherwise the rotational speed of the input drive shaft is not transferred, as intended, onto the output drive shaft but rather onto the output shaft of the auxiliary drive. In the case of a rotational speed superposition device for a motor vehicle, this would result in the fact that the driver can no longer determine the maximum angle of turn with certainty. The locking device for the optional torsion-tight coupling between input drive shaft and output drive shaft assumes a correspondingly high importance.
EP 133 84 93 A1 and JP 200330615 A introduce a device for the rotational speed superposition for a steering system in which the rotational speed superposition takes place with a strain wave gearing. The device is herein formed integrally and rotatably with the steering shaft driven by the steering wheel and includes a drive motor whose driving rotational speed is adjustable in order to obtain the desired rotational speed superposition. In the introduced solution the housing is connected with the steering shaft and consequently rotates simultaneously. For the mechanical forced coupling of the input with the output shaft of the superposition device in the event of failure, for example in the event of power outage, a positive locking analogous to a coupling toothing is proposed, which through an axial movement of the opposing coupling sides is coupled in with spring force. The coupling is controlled through the torques engaging on the gearing from the input drive shaft connected with the control wheel or the output drive shaft connected with the wheels.
A significant disadvantage of this solution comprises that the driver must rotate with the steering wheel the entire device, including the safety coupling, when controlling the motor vehicle.
Further, DE 198 23 031 A1 introduces a rotational speed superposition device with a safety coupling for the secure and reliable torque transfer in the event of failure. In the various embodiment examples in the event of failure a pin is moved through a solenoid parallel to the axis of the input drive and output drive shaft, respectively, and the gearing is blocked. In one embodiment example a row of stud holes are provided in a suitable gearing member for the reception of the pin. In a further embodiment example, in which superposition is realized with a harmonic drive gearing, the pin is brought into engagement between the elliptic rotor and parts of the housing or alternatively between the toothings of the gearing.
To operate such devices it is reasonable or even necessary to hold the safety coupling initially in the coupled-in state during the start of the motor vehicle, since, as a rule, it cannot be ensured that sufficient energy for operating the rotational speed superposition device is immediately available.
However, the solutions indicated in prior art hold a number of disadvantages. The blocking elements of the safety coupling must abruptly brake the potentially rotating rotor of the electric motor and, in the process, transfer considerable forces such that frequent coupling in and coupling out of the coupling lead to wear or the coupling must be laid out complexly and with large installation requirements. Further, the rigidities in the torque transfer are not very high. In addition, coupling in is not possible in every angular position.